1. Field of the Invention
This invention relates to a chopper-type switching regulator, and more specifically, to an improved switching regulator in its operation during transitional period being up to a stationary state after closing of a power switch.
2. Description of the Prior Art
Chopper-type switching regulators are classified into various types, one of which, for example, is of a stepdown self-excited oscillating type as shown in FIG. 1.
Referring to FIG. 1, a rectifying-smoothing circuit composed of a diode D.sub.1 and a capacitor C.sub.1 is connected to a commercial alternating-current power supply. The rectifying-smoothing circuit includes a power switch S.sub.1. A transformer T for blocking oscillation includes a primary coil N.sub.1, a secondary coil N.sub.2, and a feedback coil N.sub.3. One end of the primary coil N.sub.1 is connected to an output of the rectifying-smoothing circuit and the other end is connected to a collector of a switching transistor Q.sub.1 through a soft-start switch S.sub.2 having a current-limiting resistor R.sub.0 connected in parallel therewith. A base of the switching transistor Q.sub.1 is connected to one of the feedback coil N.sub.3 through a resistor R.sub.2 for limiting a positive feedback current and a capacitor C.sub.2 for turning ON the switching transistor Q.sub.1, and the other end of the feedback coil N.sub.3 is connected to an emitter of the switching transistor Q.sub.1, so that a positive feedback loop is composed. A diode D.sub.3 for charging the capacitor C.sub.2, the capacitor C.sub.2, and the feedback coil N.sub.3 compose a charging loop operating at the OFF period of the switching transistor Q.sub.1. The secondary coil N.sub.2 of the transformer T is connected to a flywheel diode D.sub.2 for supplying the energy stored in the secondary coil N.sub.2 to the output point B. A control transistor Q.sub.2 for by-passing a base current (positive feedback current) of the switching transistor Q.sub.1 has an emitter connected to the base of the transistor Q.sub.1 and a collector connected to the emitter of the transistor Q.sub.1, i.e. an output points B. A base of the transistor Q.sub.2 is connected through a resistor R.sub.3 to a collector of a detecting transistor Q.sub.3 which detects the voltage variation at the output point B in cooperation with voltage-dividing resistors R.sub.5 -R.sub.7 and a zener diode D.sub.4. A base of the detecting transistor Q.sub.3 is supplied with a divided voltage by the voltage-dividing resistors R.sub.5 -R.sub.7 of the voltage at the output point B, and an emitter of the same is supplied with a constant voltage provided by a resistor R.sub.4 and the zener diode D.sub.4. A capacitor C.sub.3 for smoothing the output voltage is connected to the output point B.
FIG. 2 indicates waveforms of voltages and currents at some points in the conventional chopper-type switching regulator as shown in FIG. 1. Referring now to FIG. 2, operation of the chopper-type switching regulator in FIG. 1 will be schematically described in the following. Here, let it be supposed that, in a stationary state where both the power switch S.sub.1 and the soft start switch S.sub.2 have been closed, the switching transistor Q.sub.1 has been just turned OFF. Since the collector current of the switching transistor then is zero, no current passes through the primary coil N.sub.1 of the transformer T. At this time, the transformer T has energy stored in it by the current passing through the switching transistor Q.sub.1 during its previous ON period, and this energy is dissipated in the form of electric current flowing back from the secondary coil N.sub.2 to the smoothing capacitor C.sub.3 through the output point B and the flywheel diode D.sub.2. At the same time, a current flows from the feedback coil N.sub.3 through the charging diode D.sub.3 to the capacitor C.sub. 2, whereby the capacitor C.sub.2 is charged in the polarity as indicated in the drawing. When the dissipation of the energy stored in the transformer T is finished, namely at the time point when the current passing through the secondary coil N.sub.2 has become almost zero, and accordingly the charging current to the capacitor C.sub.2 has simultaneously become almost zero, the capacitor C.sub.2 starts to discharge and this discharged current is applied through the resistor R.sub.2 to the base of the switching transistor Q.sub.1. As a result, the switching transistor Q.sub.1 is turned ON, and its collector current starts to flow. The flowing of the collector current of the transistor Q.sub.1 causes a positive feedback current supplied from the feedback coil N.sub.3 to the base of the transistor Q.sub.1 through the capacitor C.sub.2 and the resistor R.sub.2, hence the collector current of the transistor Q.sub.1 increases almost linearly, and when the current value becomes .beta. times as large as the base current value limited by the resistor R.sub.2 the switching transistor Q.sub.1 is turned OFF again. The voltage across the coil N.sub.2 during the ON period is E.sub.i -E.sub.o when the turn ratio between the coils N.sub.1 and N.sub.2 is 1:1.
In regard to the switching operation of the switching transistor Q.sub.1 being performed as described above, the relation of the output voltage E.sub.o (the voltage at B) and the input voltage E.sub.i (the voltage at A) is given by: EQU E.sub.o =T.sub.n /T.multidot.E.sub.i,
where T is the switching period and T.sub.n is the ON period. The output voltage E.sub.o at the point B thus determined is divided by the voltage dividing resistors R.sub.5 -R.sub.7 and then applied to the base of the detecting transistor Q.sub.3. If and when the output voltage E.sub.o varies, its variation causes variation in the base voltage of the transistor Q.sub.3, which in turn causes variation in the collector current of the transistor Q.sub.3, that is, the base current of the transistor Q.sub.2. With the variation in the base current of the control transistor Q.sub.2, the emitter current of the transistor Q.sub.2, that is the by-pass amount of the base current of the switching transistor Q.sub.1, varies, and in consequence, the ON period T.sub.n of the switching transistor Q.sub.1 is controlled. Then the output voltage E.sub.o is controlled according to the above equation, and thus the voltage stabilizing operation is performed.
Next, the case of a transitional period, that is, the state immediately after the closing of the power switch S.sub.1 will be considered in the following. First, it will be assumed that the soft-start switch S.sub.2 and the resistor R.sub.0 are not inserted in the circuit in FIG. 1. When the power switch is closed, a current is applied to the base of the switching transistor Q.sub.1 through a starting resistor R.sub.1 allowing a slight collector current to flow. Hence, a current flows through the primary coil N.sub.1 of the transformer T, which induces an current flowing through the feedback coil N.sub.3, that is, the positive feedback current, which is applied to the base of the switching transistor Q.sub.1. At this time, namely, immediately after the closing of the power switch, both the detecting transistor Q.sub.3 and the control transistor Q.sub.2 are OFF, and therefore all of the above-mentioned positive feedback current flows as the base current of the switching transistor Q.sub.1 through the resistor R.sub.2. Hence, this transistor Q.sub.1 will remain ON and its collector current will increase approximately linearly until either the collector current becomes .beta. times as large as the above-mentioned base current or the capacitor C.sub.2 is fully charged in the polarity reverse to that as shown in the drawing. Hence, there is the possibility under such transitional conditions that a large collector current flows through the switching transistor Q.sub.1, which current causing the transistor Q.sub.1 to be operated beyond its safety range of use and to be broken down. Therefore, the soft-start switch S.sub.2 and the resistor R.sub.0 have been provided in the circuit as shown in FIG. 1, wherein it is arranged that the switch S.sub.2 is open when the power switch S.sub.1 is closed so that the collector current flowing through the switching transistor Q.sub.1 is limited by the above-mentioned resistor R.sub.0, and the switch S.sub.2 is closed after the output voltage E.sub.o at the point B has reached a sufficiently high value.
However, if the above-mentioned switch S.sub.2 is to be opened and closed by hand, two times of switching operation becomes necessary including that for the power switch S.sub.1, and this certainly is inconvenient. The above-mentioned switch S.sub.2 may be a relay or the like that is able to be automatically closed in response to detecting the increase in the above-mentioned output voltage E.sub.o, which however, involves the defect that the circuit will become rather complicated.